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Special Troubleshooting Procedures

Preliminary Work. -The results of a study of troubleshooting methods usedin the factory showed that one type of electrical fault could not be isolated byconventional troubleshooting methods. When short circuits occurred insideor under a flatpack and on a line common to many packs, such as a B-supplyline to ground, the short could not be isolated because the only isolationbetween the sealed packs was the low resistance of the printed circuit boardpaths.The short isolation technique was developed on a printed circuit board assem-bly which had failed a production electrical test because of a short circuiton the 28-volt supply line. The 28-volt path passed adjacent to ground pathsunderneath six separate hybrid circuits and also entered these hybrid circuitsto become part of the internal circuitry. There were many possible locationsfor the short. The only isolation between these possible locations was theresistance of the copper circuit paths between these points. The obviouschoice seemed to be current detection, since workable current can be passedthrough low resistance circuits with negligible power dissipation.The method chosen for isolating the short circuit was to apply a low voltageto the 28-volt line and try to identify the paths carrying current; then, bystrategically moving the current application points and observing those pathsthat carried current, to plot a physical location of the short circuit. Voltageand resistance measurements were not considered desirable because of theobvious difficulties encountered in measuring voltage drops or resistancein short lengths of copper circuit paths. Noncontacting current probes arecommercially available, but design changes would be required to adapt suchprobes to printed circuit board paths. Since no totally-adequate probe wasreadily available and redesigning a probe would be time consuming and ex-pensive, alternate methods of detection were investigated.X-ray techniques were considered, but were not chosen as a primary tech-nique principally because of analysis time and possible additional equipmentneeds. Thermal indicators and liquid crystal techniques were then con-sidered. Because speculation about the location of the short could be replacedwith exact knowledge of the location of the short, several hybrid componentsworth about $250 each were saved.Short-Circuit Detection With Liquid Crystals. By using the liquid-crystaltechnique, hidden short-circuit faults (inside flatpacks, hybrid circuits, andunder large components) may be isolated by identifying current-carryingcircuit paths with a liquid-crystal temperature indicator. The liquid crystalsare mixed to display color through a temperature range of approximately 3 C.Below and above this range, the crystals are transparent. The mixture isadjusted so the lower end of the 3 C range is at ambient room temperature.The color at the lower end of the range is red and at the higher end it is blue.